Since a stepping motor has inertia in a rotor thereof, even if there is no load in the stepping motor, the rotor rotates in steps by the number of pulse inputted and then the stepping motor stops at a predetermined angle after damping the amplitude of reciprocating motions thereof with overshooting and undershooting angles. Magnetic flux is generated from excited pole teeth of a stator yoke during such a transient response. When the rotor vibrates in such a way as to cut off the magnetic flux, a change in magnetic flux is generated which corresponds to a deflection amount of the rotor relative to the stabilizing point. As a result, a back electromotive voltage is generated. This back electromotive voltage influences an exciting current to thereby generate a vibrating current. The rotor is driven by a voltage resulting from adding a vibrating voltage to a supply voltage. Reverse effects appear in settling time and stopping position accuracies due to stiffness characteristics of the stepping motor, frictional resistance in an interior of the motor and the like, in addition to the vibrating voltage.
The stiffness characteristics are a relationship between quantity of load applied externally to an output shaft of the stepping motor and an angle through which the stepping motor is displaced by the load when the stepping motor is excited by a direct current, and denote a restoring force to the stabilizing point. According to the stiffness characteristics, when a load of the stepping motor attempts to be positioned, since the accuracy becomes better in the event that a displacement angle θL with respect to a load torque TL is smaller, the stepping motor is desired to have a larger maximum static torque (a holding torque) TM.
According to the stiffness characteristics, when the position of the rotor deviates from the stabilizing point in the excited state, a torque is generated in the rotor in an opposite direction to that in which the torque has been generated until then. In addition, also in the event that excitation of the excited pole is cancelled, when the rotor is caused to deviate from the stabilizing point, a torque attempting to attract the excited pole (a detent torque) is generated, so as to attract the rotor back. In addition, when a frictional resistance Tr is present, since the rotor cannot rotate without a torque which can overcome the frictional resistance Tr, the rotor has a dead zone equal to an angle of rotation +/−θ due to the frictional resistance Tr, whereby a static angle error equal to the dead zone is generated. When discussing here about dead zone, a dead zone for a detent torque is larger than a dead zone for a holding torque. This is due to a difference in inclination of torque characteristic curves. The angle error becomes larger when attempting to hold the rotor only by a detent torque which is smaller in magnitude of torque.
There are the following methods as stopping position control methods.
(1) There is a control method in which by using a motor in which a holding torque stabilizing point and a detent torque stabilizing point are made to match each other, energization is discontinued after driving is completed, so that a rotor is to be held by a detent torque thereafter. In this case, since a detent torque is much smaller than a holding torque, an angle deviation corresponding to the static angle error θ becomes very large. Consequently, when adopting the aforesaid stepping motor control method, energization needs to be discontinued after the oscillation of the rotor has sufficiently been dampened by the holding torque working during energization, so that the rotor is then held by the holding torque (refer to Patent Document No. 1).
(2) There is a control method in which a magnetic pole in a stator, which corresponds to the angle of rotation at which a rotor is to be stopped, is excited, and the excitation is cancelled when the rotor is attracted to the magnetic pole and stopped, so that the position of the rotor is held only by a detent torque thereafter. However, since there is present inertia in the rotor, it is inevitable that oscillations continue to repeat relative to the rotational direction of the rotor until the rotor is brought to a complete stop. Then, although after oscillations have sufficiently been dampened by a holding torque, the rotor is held by detent torque, in a normal using state, ample time has to be spent implementing holding i.e., energization so that oscillations are dampened sufficiently so as not to cause neither positional deviation nor step-out. Consequently, energization volume is increased and control time is extended. In addition, when used under abnormal environment in terms of vibration, temperature, humidity and the like, oscillations especially become large and cannot be suppressed properly within a holding time which is determined by anticipating holding under the normal conditions, leading to step-out. (Refer to Patent Document No. 1).
(3) There is a control method for detecting the oscillating state of a rotor by a detector to control the duration of energizing a winding to obtain a holding torque to suppress the vibration of the rotor based on a detection output of the detector.
According to this control method, the vibration of the stepping motor is electrically detected when controlling the stopping of the stepping motor, and the excited magnetic pole at the stopping position continues to be excited by the duration corresponding to the detection output relating to the vibrating state of the rotor in order to hold the rotor by means of energization until the oscillating amount decreases below a predetermined amount. Thereafter, the excitation is discontinued so that the holding of the rotor is switched to holding by a detent torque. By this configuration, the required minimum amount of holding torque is given, and the holding of the rotor is switched to the holding by the detent torque at the stage where the holding by the detent torque suffices. (Refer to Patent Document No. 1).
The control method described in the above (3) provides the following issues.
Since inertia is present in the rotor, it is inevitable that vibrations repeat relative to the rotational direction of the rotor until the rotor stops completely. Then, the rotor is held by the detent torque after the vibrations are dampened sufficiently by the holding torque. However, in a case where the load is a vibration generating weight which has a large mass, the magnitude of vibration that continues to be generated until the rotor stops becomes extremely large when compared with a load which is considered to have nothing to do with generation of vibrations (such as a weight which is not used substantially for generating vibrations), and although vibrations are averaged out while the rotation speed is high, vibrations tend to be become conspicuous one by one as the rotation speed decreases. Because of this, there are caused the following issues; it is difficult to detect a condition where the vibration amount becomes equal to or lower than a specific amount by detecting the vibration amount, and as a result of continuation of the holding control over a long period of time, the rotor suddenly stops immediately after the action on the detent torque, or the rotor comes to balance with the weight constituting the load to stop in a position which largely overshoots or undershoots the stabilizing position.
Patent Document No. 1: JP-A-2-136098